1. Field of the Invention
The present invention relates to wireless communication, and more specifically to a method of sending data using multiple antennas.
2. Discussion of the Related Art
A Multiple-Input Multiple-Output (MIMO) has recently been in the spotlight in order to maximize the performance and communication capacity of a wireless communication system. MIMO technology is a method which breaks away from technology using one transmission antenna and one reception antenna and can improve the transmission efficiency of transmission/reception data by adopting multiple transmission antennas and multiple reception antennas. An MIMO system is also called a multiple antenna system. MIMO technology is the application of technology for gathering and completing data pieces received by several antennas without being dependent on a single antenna path in order to receive one entire message. Consequently, the data transfer rate may be improved in a specific range or the range of a system for a specific data transfer rate may be increased.
MIMO technology includes a transmission diversity, spatial multiplexing, beamforming, and so on. The transmission diversity is technology for sending the same data through multiple transmission antennas in order to increase transmission reliability. The spatial multiplexing is technology for sending data at high speed without increasing the bandwidth of a system by sending different data through multiple transmission antennas at the same time. The beamforming is used to increase the Signal to Interference plus Noise Ratio (SINR) of a signal by applying a weight according to a channel state in multiple antennas. The weight may be represented by a weight vector or a weight matrix and called a precoding vector or a precoding matrix.
The spatial multiplexing includes spatial multiplexing for a single user and spatial multiplexing for multiple users. The spatial multiplexing for a single user is called a Single User MIMO (SU-MIMO), and the spatial multiplexing for multiple users is called Spatial Division Multiple Access (SDMA) or a Multi-User MIMO (MU-MIMO). The capacity of an MIMO channel is increased in proportion to the number of antennas. An MIMO channel may be divided into independent channels. Assuming that the number of transmission antennas is Nt and the number of reception antennas is Nr, the number of independent channels Ni is Ni≦min{Nt, Nr}. Each of the independent channels may be said to be a spatial layer. A rank is the number of non-zero eigenvalues of an MIMO channel matrix and may be defined as the number of spatial streams that can be multiplexed.
MIMO technology includes a codebook-based precoding scheme. The codebook-based precoding scheme is a method of selecting a precoding matrix which is the most similar to an MIMO channel, from among predetermined precoding matrices, and sending a Precoding Matrix Index (PMI). This method can reduce the overhead of feedback data. A codebook consists of codebook sets which can represent spatial channels. In order to increase the data transfer rate, the number of antennas has to be increased. With an increase of the number of antennas, a codebook has to consist of a more number of codebook sets. If the number of codebook sets increases according to the increased number of antennas, not only the overhead of feedback data may be increased, but also there is a difficulty in designing the codebook.
There is a need for a method to which a codebook-based precoding scheme can be efficiently applied in a multiple antenna system having an increased number of antennas as compared with the existing number of antennas.